Electrical discharge device



Sept. 20, 1949. c. B HORSLEY ELECTRICAL DISCHARGE DEVICE Filed Nov. 26,1945 2 Sheets-Sheet l B 242 m $112; Ms

I ATTORNEYS ar/z IL.

Sept. 20, 1949. c. B. HORSLEY ELECTRICAL DISCHARGE DEVICE 2 Sheets-Sheet2 Filed Nov. 26, 1945 Patented Sept. 20, 1949 TENT QFFICE.

ELECTRICAL DISCHARGE DEVICE Caperton B. Horsley, Stamford, Conn.,assignor to Machlett Laboratories, Inc.,- Springdale,

Conn., a corporation of Connecticut Application November 26, 1945',Serial No; 630,812

2'7 Claims.

This invention relates toelectron discharge devices and is concernedmore particularly with a device ofthat type which comprises an anode andcathode of novel construction and arrangement, together with a new meansfor deflecting electrons issuing from the cathode and focusing them uponthe face of the anode. The cathode includes an incan'descible filament,which is of large size and capable of producing a large quantity ofelectrons, and the cathode encircles the anode and may be so mountedthat the cathode and the supporting means therefor are wholly outsideany hemisphere subtended by the plane of the face of the anode. Theinvention may be embodied to especial advantage in tubes for thegeneration of X-rays, which are superior to prior X-ray tubes in thatthe total X-radiation produced is available for use, so that the newtubes are peculiarly adapted for applications in which a large area of aspecimen is to be irradiated, as, for example, for therapeutic purposes,for the production of photochemical changes, etc. As X-ray tubesconstructed in accordance with the invention make use of all thefeatures thereof, examples of such tubes will be illustrated anddescribedin detail for purposes of explanation.

Up to the present, X-r-ay tubes have commonly been so constructed thatonly a small part of the total radiation generated can be used. Theradiation issues from the target in all directions within the 180 solidangle or hemisphere subtended by the plane. of the target face and atanode voltages of the. order of 50- kv. or less, the intensity of theradiation is substantially uniform throughout that hemisphere. However,a large part of the radiation is eclipsed by the cathodev and itssupporting structure, which lie within the hemisphere ofradiation, and,in addition, it has been the common practice tov provide windows ofrelatively small area for the egress of the rays, so that the availableradia-. tion is still further reduced. For diagnostic and radiographicapplications, in which information is provided as a result ofmodification of the X-ray beam caused by difierent-ial absorption by:the objects through which it, passes, the. utilizae tion. of only asmall: part of the radiation is satisfec ory. and that is true also withrespect to.

industrial; uses of X-rays for ascertaining the internal structure ofspecimens or for crystal: analysis. For applications such as theproduction of photochemical reactions; 8,110., in which. the. beamproduces. changes the object being;

ed, t e; impo s b lity of util z ng he total.

radiation generated is an objectionable feature of present tubes.

An X-ray tube embodying the present. invention is adapted; to. produceX-rays of high intensity, because of the vlargefathode employed, and theentire amount of radiation produced may be put to use, because thecathode and its sup.- porting means are so.-constructed and arrangedrelative to the anode that they lie Wholly outside the hemisphere ofradiation leaving the target surface and, thus,ydo. not. have theeclipsing effeet, as in. prior X-ray tubes. In addition, the window ofthe new tube is. of such size, shape, and location, with relation to theanode and cathode, that it deflects. theele'ctron's leaving the cathodeand focuses them upon the anode face and it also permits the egress ofall the X-rays generated.

For a better understanding of the invention, reference may be had to.the accompanying drawings, in which:

Fig. 1 is a View in longitudinal section of one form of X-ray tubeembodying the. invention;

Fig. 2-is a sectional view on line 2.:2 of Fig. 1;

Fig. 3 is. a sectional view'on an enlarged scale on the line 33 of Fig.2;

Fig. 4 is a longitudinal sectional view through an X-ray tube ofmodified construction, which is provided with means for thev irradiationof materials;

Fig. 5 is a sectional view on the line 55 of Fig. 4';

Figs. 6, 7, 8, and 9. are sectional views through the anode end of thenew X-rray tube, showing different constructions producing different.paths of travel of the, electrons. from the filament to the target;

Figs. 6a, 7a, 8a, and 9a. are views showing the focal spots produced bytravelof' the electrons along the paths illustrated in Figs. '6', 7, 8,and 9, respectively;

Fig. 10 isa longitudinal sectional'view through an X-raytube ofmodified.- construction;

Fig. 11 is an end view of:.the tube shown in Fig. 19;and '1' Fig. 12 isa fragmentary longitudinal sectional view of another form of the newX-ray tube.

The X-ray tube illustrated in Fig. 1 comprises evacuated envelopecomprising a re-entrant section of glass, united bya metal sleeve 2,!to. a cylindrical metal section 22. The section 22 is formed at its openend with an internal circumferential; channel inwhich is seated the rimor base ofa window 23, which is of dished or dome; formation. The windowis preferably made of beryllium or the beryllium alloy.

a metal fitting 36.

of Claussen Patent 2,036,592, issued December 29, 1942, and the rim ofthe window is brazed in the channel with a vacuum-tight seal. A tubularshield 24 is mounted at the inner end of the section 22 to projectbeyond and protect the metal-glass seal between the glass section 20 andthe metal sleeve 2| against electron bombardment. V I I A cathode'in theform of a metal ring 25 is mounted within the outer end of section 22-in a circumferential seat provided for the purpose. This ring is formedwith a circumferential channel 26 facing outwardly and a filament2'|,'which may be helically wound on a plain wire,.is 'mount-. ed withinthe channel on aplurality of metallic stems 28 spaced angularly aboutring-25 and extending through insulating bushings 29 seated in openingsthrough the ring leading to the channel. The filament is, accordingly,substantially circular in shape and it lies in a plane. A conductor 30,adapted-to be connected to a source of filament current and sealedthrough the wall of glass section 20, extends through one of thebushings 29 and supports and is connected to one end of the filament.The other end of the filament is connected toa stem 3|, which is mountedinan opening through ring 25 leading to channel 26 and is in electricalcontact through the ring with the metal envelope section .22 and window23. The section 22 is grounded in any appropriate fashion and thecathode is likewise at ground potential.

A hollow metallicanode shank 32 is mounted within the envelope byattachment to a sleeve 33 sealed in the end of the re-entrant portion ofglass section 20. The shank carries a target disc 34 on its inner endand the disc faces the interior of the window and is substantiallyconcentric therewith. The outer or target surface of the disc issubstantially plane and lies substantially parallel to the plane, inwhich the filament lies, and the target surface lies closer to thewindow than the plane of the filament. A metal tube 35 extends into there-entrant end of the envelope section 20 and is seated in an internalchannel in the end of the anode shank 32. At its outer end, the tubeenters A metal tube 31 of smaller diameter than tube 35 extends throughthe fitting 36 and tube'35 concentrically withthe latter and into theanode shank to terminate close to the end wall thereof behind the targetdisc. A line 38, connected to a source'of coolingfluid, delivers coolingfluid to tube 31 to be conducted thereby through the anode'shank anddischarged against the wall at the back of the target. The fluid thenreturns through the space between tubes 35 and 31 to be dischargedthrough line 39. The anode may be connected to a source of highpotential through a conductor 40 leading to fitting 36, and lines 38 and39 contain sections of insulating tubing of substantial length toinsulate the anode.

In the operation of the tube, the anode is main tained at ahigh'pos'itive potential while the oathode and the windoware at groundpotential. The

are of greater'radius, so that the electrons from' that the paths of theelectrons are of relativelyshort radius, and the focal spot produced isof the shapeillustratedat 42 in Fig. 6a. When the space between thetarget and window is substantially increased, as shown in Fig. 7, theelectron paths 7 one point'on; the filament may pass across the face ofthe target to strike the target surface beyond its center. Under thoseconditions, the focal spot isofthe shape indicated at 43in Fig. 7a. Boththe focal spots- 42, 43 have a recess becaus of the gap in the source ofelectrons represented by the I space between the ends of the filament.By proper spacing of the window and target surface, as

inner wall 25a of the channel 26 in the cathode 1 projects outwardlybeyond the end of the outer shown in Fig. 8, the electrons may be causedto impinge at thecenter of the targetto produce a substantiallycircularfocal spot 44, as shown in Fig. 8a, and a similar 'spot 45(Fig.v 9a) may be produced byslightly increasing the spacing between thewindow and thetarget surface, as

shown in Fig. 9. I L

In the new tube, the cathodeand the support therefor may lie whollyoutside any hemisphere subtended by the-plane of the target surface andthe window isof such size and form that its base is outside thathemisphere. Accordingly, noneof the radiation is intercepted and all ofit may issue through the window.

The new X-ray tube is illustrated inFig. 4 in a form suitable forthetreatment of'materials to produce changes therein, as, for example,the destruction of bacteria for the sterilization of food products, theproduction of photochemical changes, etc. 'For such purposes, the tubeis provided with a circumferentialfiange 45 adjacent. thewindow and acasing 41 is mounted on the flange to enclose the window. The casing hasa circumferential'flange 48, which may be secured to flange 46, byscrews 49, a gasket 50' being interposed between the flanges to make aliquidtight joint, before they are secured together. Casing 41 has ahollow stem 41a extending coaxially' with the domed window and a-tube 5|is mounted in the end of the stem and extends concentrically therewithto terminate close to the window; 4 A concave'plate 52 is mountedontheinner end of 'tube 5! to lie. close to the window and the edge of theplate terminates close to the surface of flange 46, so that the plate,window,.and flange define a thin chamber of'substantial area. A spiralpartition 53'may be mountedon the inner surface of plate 52 toextendclose to the surface of the window and this partition may be providedwith baffles 54 lying between its convolutions. The partition, togetherwith plate 52 and the'window, defines a spiral'passage having anentrance end .toiwhich' tube 5| leads, the discharge end of the passagelying at the rim of the plate 52.

In theme of the tube described, the material to be irradiated'may be agas or liquid or in the form of fine particles-suspended in a gas orliquid.

The fluid is supplied through a line 55 connected to theouter end of thestem fla, and enters tube 5| to be conducted to the chamber betweenplate 52 and the window. The material flows over the surface of thewindow through a prolonged path to escape beneath the edge of plate 52.The material 'then flows back between the outer surface of the plate andthe inner wall of themeing into the space within the stem 41a around theouter .surfacelof tube 51:; .Ihe material-escapesthrough .:.a lineleading from the interior of stemifla. r

The degreeaof uniformity with which the material is irradiated :depends-:,on :the "thickness of the film.of;material;flowing over the surfaceof the .window .in that vzthe layers in th filmnea-r theWiHdOWJfibSQI'bTSDIIlG. of the radiation and thus shield the imaterial.in.,:1.&yers,-remote from the window. For some purposes-it ,is notnecessary toprovide the-spiral partition -3 and :the baffles5j4,abnt,:in theseventrthat the partition and baiiles are :not used, itis generally necessary to amount plates-52.01056 :tothe outersu-rface ofthe window so :that the film of material between the plate and .windowisrelatively thin. When the partitioniand bafiles areemployed, thefilm'ofmaterial may .be thicker, =since:it is not only exposed to theradiation for a :longer period ;but also the material :flo-w-siturbulently .over the-surf ace of the window andall partsofthematerialthus receive equal treatment.

The X-ray ztubeiliustrated in Fig. is ,1Providedxwith means by which therelative positions of the window .and target surface may be varied tovary the focusing :action of the window and, thus,1produce,focal spotsof different forms. l he tube includes :az-re-entrant glass sectionconnected by .a :metal sleeve 2l sealed in the end thereof, :to :acylindrical metal section .22 which is grounded in any suitable way. Anannular cathode isseated within the outer end of secti-on 22.a-ndahelicalyfilament 2'5 is mounted in a circumferential channelin theface of the cathode. :Qne end of thefilament is'connected to a stem 3.0,which extends through an insulating bushing in an mlenin fin the cathodeand is connected to ;,a conductor .51 leading through an opening in.section 22'. "A metallic sleeve 58 .is sealed within :theopening'andthe conductor extends :through .the sleeve and is sealed through thewall ,of a glass .cupiS-sealed to the end of the sleeve. 211 c conductoris connected outside .the envelope to a sourceof filament current.

A domed window 23' which is preferably of heryllium orrof the Claussenalloy,-- closes the-end of section 2.2. The window is provided with alateral :fiange -60 secured against one face of a ring 61, which closelyencircles the section .22 and is movable thereon. Another ring 6.2encircles section 22' and is secured to :a circumferential flangeProjecting from the outer surface of the section. The ringsare connectedby angularly spaced screws 64 which extendlthroughopenings in ring '61and are threaded into ring :82, A spring .65 encircles each screw andbearsagainst the opposed faces of the rings at its ends. The springstend :to ,force ring 6! outwardly away from ring 52 andzthe screws .64limit such outward movement. -A plurality of screws t5 threaded throughring '61 bear against the outer face ,of ring 62 and serve as a means bywhich ring -61 may be moved toward and away from ring 62 within :theilimits permitted.

A metallic bellows r61 encircles section 22 of theenvelope and its endsare secured, respectively, to opposed faces of ring '6! and flange 63. Ametal tube 68 is mounted around the outer surface of the windowad-jacentflangebi) and a cooling fluid maybe passed through the tube to helpdissipate the filament energy.

The tube of ,10 contains an anode 32' having a flat target :disc 34 atits outer end, the anode being mounted on a sleeve 33 sealed in the re-entrant end portion of the envelope section 2.0. :Cooling .fiuid isintroduced :into the anode through ;a metalifitting;3,6'-.anda tube 31"leading therefrom i-in'to the anode to terminate close "to the end 'wallthereof. The fluid returns through the space within the anode outsidetube Y31 and through arspace between-that tube and :a metal tube 35which encloses tube 31 and extendsjfrom the :outer end of the anode to:fitting 155?. The discharged fluidleaves the fitting through a line391. The anodezisconnectled to a source of high potential through aconductor 4107, which may :be connected to fitting .3162

Therelation between :the window andthe target surface of thetube shownin Fig. :10 may be varied by turning the screws :66 and thus moving.ring BI .and the window toward or away from the stationary ring '62,while .the bellows 6'5 contracts or expands. -The .bellows forms part ofthe :tube envelope enclosing the evacuated space and,;since there .is apossibilityrof leakage at the joints .between the ends :of the bellowsand the parts to which they are connected, the glass section -20 isprovided with a hollow neck 59-, which may .be connected to a pumpoperable during the use of the tube to maintain the vacuum therein.

Instead of varying the focusing eifectof the window by altering therelative positions of the window and target surface, the same resultsmay be obtained by varying the potential maintained on the window. Forthis purpose,the construction illustrated in Fig. 12 may be employed.The tube shown in Fig. lz-include's an-envelope having a metallicsection 22a, within which is the cathode 25a and the anode 152a providedwith a target disc 34a. The tube includes a beryllium window'z'sao'fdomeshape, the rim of which enters .and is brazed to a ring if! of Monelmetal. Ring 'll! is soldered to the'outer surface of a cup 1'! made ofthe alloy .known commercially as Kovar. The rim :of cup ill is sealedinone .end ofa glassring :lZZ andtherim of a.second"Kovar' cup 1:3 issealed .in the other end of the glass ring. At its other end, the cup 73lies against and is soldered. to the face of a shoulder "14 formed .inthe tube section 22a. The glass ring insulates the window from thecathode and tube section 22a and the window is connected through arheostat J 5 to the negative sideof a battery 16, the positive side ofwhich is grounded. With this arrangement, the Window maybe maintained ata variable negative potential relative to the cathode so as to have avariable focusing effect upon the electrons leaving the cathode andtraveling to the surface of the target.

In the several X-ray tube constructions illustrated, the envelope isformed partly of metal and partly of glass, and the window is describedas being made of beryllium or of the Claussen alloy, these metals beingchosen because of their low X-ray absorptive power. For some purposes,however, it is satisfactory to 'form the tube with a glass envelope, ofwhich a domed portion of a thickness of the order of -1 m. m. forms thewindow. Such a window will collect a-charge, which will assistinfocusing the electrons, although provision shouid be made in the usualway to prevent the accumulation upon the window of so great a chargethat puncture may result. Also, in tubes to be operated at voltages ofkv. and higher, the window maybe made of any suitable metal, .such ascopper, aluminum, or stainless steel, the latter bein desirable becauseof its resistance to corrosion. In discharge tubes other than for X-raypurposes, the focusing means will lie within the hemisphere subtended.by the plane 7 of the target surface and may be mounted wholly withinthe envelope or may form part of the en,- velope, as desired. In suchdevices, the X-ray absOrption characteristics of the metal used for thefocusing means is unimportant and, therefore, beryllium or the Claussenalloy need not be employed.

In the tube shown in Figs. 1 and 10, the oathode and its supportingmeans he wholly to the rear of a plane through the target surface, sothat they are entirely outside the hemisphere of radiation. In the tubeshown in Fig. 4, the extreme end of the cathode projects into thehemisphere of radiation to a small extent, but substantially all of theradiation is available. In all devices embodying the invention, thecathode is unusually largeso that a large quantity of electrons isproduced and X-ray tubes embodying the invention are, accordingly,adapted for the production of high intensity X-radiation. Also, in suchX- ray tubes, the area of the window is such as to permit substantiallyall the radiation "to pass therethrough. V 1

I claim:

1. An X-ray tube which comprises an evacuated envelope, an anodeincluding a substantially plane target surface within the envelope, anda cathode including a filament within the envelope and in such relationto the anode that electrons from the filament travel to the faceof thetarget, the cathode lying substantially entirely outside any hemispheresubtended by the plane of the target surface and the envelope includinga window relatively more permeable to X-rays than the remainder of theenvelope and of such shape and location relative to the target surfacethat substantially all the radiation emanating from the target facepasses through the window.

2. An X-ray tube which comprises an evacuated envelope, anode andcathode means within the envelope, the cathode means including afilament, the filament lying coaxially of the anode and in such relationthereto that electrons travel along curved paths from the filament tothe.

potential thereof and acting on the electrons issuing from the filamentto assist in causing them to travel along said curved paths to theanode. V

3. An X-ray tube which comprises an evacuated envelope, an anodeincluding a substantially plane target surface within the envelope, acathode including a filament within the envelope, the filament havingsubstantially the form of a circle about the center of the targetsurface and of larger diameter than the target surface and the cathodebeing in such relation to the anode that all the electrons from thefilament that travel to the anode move along curved paths, a windowforming part of the envelope and of larger diameter than the filamentfor egress of the rays.

4. An X-ray tube which comprises an evacuated envelope, an anodeincluding a substantially plane target surface within the envelope, anda cathode including a filament within the envelope, thefilament being ofsubstantially closed form with its ends close to one another andsubstantially concentric with the target surface, the filament definingan area of greater transverse dimensions than the target surface and theoathode being in such relation to the target surface cathode including afilament within the envelope,

the filament lying spaced laterally from theprojecting area of thetarget 'surface'andsubstantially entirely outside any hemispheresubtended by the'plane of the target surface.

6. An X-ray tube which comprises an evacuated envelope, an anodeincluding a substantially. I

plane target surface within the envelope, a cathode including a filamentwithin the envelope, the cathode lying substantially entirely outside ay hemisphere subtended by the plane of the target surface, and a windowfor egress .of X-rays formingpart of the wall of the envelope, thewindow lying within said hemisphere. I

'7. An X-ray tube which comprises an evacuated envelope, an anodeincluding a substantially plane target surface withinthe envelope, acathode including a filament withinthe envelope, the cathode lyingsubstantially entirely outside any hemisphere subtended by the plane ofthe target surface, and a window for egress of X-rays form'- ing part ofthe wall of the envelope, the window being of dished shape and havingthe major portion of its area within said hemisphere.

8. An X-ray tube which comprises an evacuated envelope having a windowfor egress of the rays, an anode within the'envelope having asubstantially plane target surface facing the win- 9. An X-ray tubewhich comprisesan evacuated envelope having a metallic window for egressof the rays, the window being of dished shape,

an anode within the envelope having a substantially plane targetsurface, the rim of the Window lying adjacent the'plane of the targetsurface, and. a cathode including a filament within the envelope, thefilament lying substantially entirely outside any hemisphere subtendedby the plane of the target surface and being electrically connected tothe window. 7 a

10. An X-ray tube which comprises an evacuated envelope having ametallic window for egress of the rays, an anode having a target surfacewithin the envelope, and a cathode having a filament within theenvelope, the filament being substantially concentric with the targetsurface and defining an area greater than that of the target surface,the window being connected electrically to the filament and beingadapted to act as a focusing means for the'electron beam leaving thefilament. 7

11. In an X-ray tube, the combination of an anode having a substantiallyplane target sur-' face and a cathode including a filament lying anodehaving a substantially plane target surface 7 and a cathode including afilament lying substan-' tially in a plane parallel to the plane of thetar-. get surface, the filament lying spaced laterally from the targetsurface. r

13. An X-ray tube which comprises an evacu V 9 atedenvelopehavi'nga-metallic-window for egress of the rays, an anode;having a target surface within the envelope; a cathode having a filamentwithin the envelope, the window being connected electrically to thefilament and adapted to focus the electrons passing from. the filamentto the target surface, and means for varyin the relative positions ofvthe target, surface and the window to vary the focusing action of'thelatter and produce a selected focal spot. I

14. AnX-raytube which: comprises an evacuated envelope havinga metallicwindow for egress of the rays, an anode having a target surface withinthe envelope, a. cathode having a filament within the envelope, thewindow being connected electrically to the filament and adapted tofocusthe electrons passing. from thefilament to the target surface,,andmeans,fo r varying the position of the window relative to the targetsurface to vary the focusing action of the window and produceas'electedfocalspot.

15. An X-ray tube-which comprises an evacuated envelope having ametallic window of outwardly dished form" at one end, an anode withinthe envelope having a target surface facing the concave surface of thewindow, a cathode within the envelope having a filament lyingsubstantially concentric with the anode and spaced laterally from thetarget, the window and filament being electrically connected, wherebythe window is adapted to focus electrons to cause them to travel along acurved path from the filament to the target surface, and means foraltering the relative positions of the window and target surface to varythe focusing action of the window.

16. An X-ray tube which comprises an evacuated envelope, an anode withinthe envelope having a target surface, a cathode within the envelopehaving a filament spaced laterally from the target surface, and focusingmeans adjacent the filament for directing electrons from the filament inthe same general direction as the axis of the anode and away from thetarget surface, the envelope having a wall acting as an additionalfocusing means for the electrons between the filament and the targetsurface.

17. An X-ray tube which comprises an evacuated envelope, an anode Withinthe envelope having a substantially plane target surface, a cathodewithin the envelope having a filament, and a focusing element having achannel surrounding the target surface and offset laterally therefrom,the channel facing in the same direction as the target surface and thefilament being mounted within the channel, the envelope having a walllying close to the curved path of the electrons between the filament andtarget surface and assisting in directing the electrons along that path.

18. An electron discharge tube which comprises an anode having asubstantially fiat face, a cathode including a filament surroundin theanode and lying in a plane parallel to the face of the anode andsubstantially entirely outside any hemisphere subtended by the plane ofthe anode face, means forming part of the cathode for directingelectrons from the filament into such a hemisphere, and means withinsaid hemisphere for deflecting the electrons and focusing them onto theanode face.

19. An electron discharge tube which comprises an anode having asubstantially fiat face, means lying substantially entirely outside anyhemisphere subtended by plane of said face for emitting a hollow beam ofelectrons traveling into the '14) hemisphere, and rneanis= within such ahemisphere for defiecting'the electrons to ca-use them to form a beam ofsmaller crossf-section striking the anodeface;

20. An electrondischarge' tube which comprises an evacuated-envelope; ananode withinthe envelopehaving a substantially; flat" face; means withinthe envelope lyingsubstantially entirely outside any hemispheresubtended'by the plane of said faceforemittingahollow beam of electronstraveling-intothe hemisphere; means within such a hemisphere"adaptedt'o-be maintained at a potential-suchthat said means deflect theelectrons and cause therrrto strike'theanodeface, and meansfor-'varying-thepoten-tialon saiddefiecting means. Y

21. Anelectroh discharge tube which comprises an evacuated envelope, an;anode within the envelope having a" substantially fiat face, meanswithin the envelope lying substantially entirely outside any hemispheresubtended" by the-plane of said face for -emitting-ahollow beam ofelectrons travelinginto -the-her'nisphere, and means forming part of thewall of the envelope for deflecting the electrons: and-causing. them tostrike the anode face. I

22. An X-ray tube which comprises an evacuated envelope, an anode withinthe envelope having a substantially flat face, a cathode insid theenvelope encircling the anode, the cathode lying substantially entirelyoutside any hemisphere subtended by the plane of the anode face andbeing operable to project a hollow beam of electrons into thathemisphere, a metallic window forming part of the envelope for egress ofthe X-rays, the window being insulated from the cathode, and means formaintaining the window at a potential such that it is effective todeflect the electrons entering such a hemisphere and cause them tostrike the anode face.

23. An electron discharge tube which comprises an anode having asubstantially plane face, a cathode including a filament surrounding theanode and lying substantially in a plane parallel to the plane of theanode face and substantially entirely outside any hemisphere subtendedby the plane of the anode face, and an envelope enclosing the anode andcathode, the envelope having a portion of domed shape adjacent the anodeand cathode acting to focus electrons emitted by the filament upon theanode face.

24. In an electron discharge tube, a cathode comprising an annularelement having a channel in one of its faces which extends transverse tothe axis of the element, the channel extending coaxially with theelement and the inner wall of the channel terminating in a plane lyingoutward beyond a plane through the end of the outer wall of the channel,and a filament of generally circu- 60 lar form mounted within thechannel.

25. An X-ray tube which comprises an evacuated envelope, an anode havinga target surface within the envelope, and a cathode including a filamentwithin the envelope, the filament en- 65 circling the axis of the anodeextending through the target surface and lying in such relation to theanode that electrons from the filament travel to the target surface, thecathode lying substantially entirely outside the path of any radiation70 issuing from the target surface in the operation of the tube and thefilament lying wholly outside the projected area of the target surface.

26. An X-ray tube which comprises an evacuated envelope, an anode havinga target surface 75 within the envelope, a cathode including afilamentwithin the envelope, the filament encircling the axis of theanode through the target surface and the cathode lying substantiallyentirely outside the path of any radiation issuing from the targetsurface in the operation of the tube, and means for-deflecting electronsemitted by the filament to cause them to strike the target surface, saidmeans including a member 1ying in the path of'said radiation and meansfor maintaining the member at a negative potential.

27. In an electron discharge tube, the combination of an anode, acathode structure including a filament of generally circular form, andapair of focusing means for causing electrons emitted by the filament toconverge along curved trajectories toward a central axis normal to theplane of the filament and to form a beam of substantially circularcross-sectional outline directed toward the anode, said beam having adiameter substantially less than that of the filament, one of saidfocusing means comprising a member having an annular focusing channelcoaxial with V the anode, in which the filament is mounted, and

the other of said focusing means including a concave member lyingcoaxial with said first member and with its concave surface adjacent thepath 12 of electrons leaving the filament and efiective to deflect themtowards the anode. a V cA RToN BFHQRSLEY;

REFERENCES cmnff v a The following references'are of recor'd'in the fileof this patent: v UNITED STAT ES PATENTS #3, pages 59-63,'Mar. 1943.

